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Speakers

The Process of Building Speakers

The Process of Building Speakers- A Magnetic Process

Sound travels in waves of air pressure fluctuation, and we hear sounds differently depending on the frequency and amplitude of these waves. Microphones translate sound waves into electrical signals, which can be encoded onto CDs, tapes, LPs, etc. Players convert this stored information back into an electric current for use in the stereo system.

 

A speaker is essentially the final translation machine -- the reverse of the Microphone. It takes the electrical signal and translates it back into physical vibrations to create sound waves. When everything is working as it should, the speaker produces nearly the same vibrations that the microphone originally recorded and encoded on a tape, CD, LP, etc.

 

We know that in order to get air molecules to vibrate, some other object must vibrate first to get the air moving.  It might be a tuning fork, or vocal cords, or in the case of a speaker, a flexible material that makes up what is known as the cone, or diaphragm. The cone is usually made up of paper, plastic or very thin metal.

Stereos produce electrical currents that need to be changed into vibrations. The part of the speaker that does this is called a voice coil. The voice coil is a basic electromagnet. An electromagnet is a coil of wire, usually wrapped around a piece of magnetic metal, such as iron. Running electrical current through the wire creates a magnetic field around the coil, magnetizing the metal it is wrapped around. The field acts just like the magnetic field around a permanent magnet: It has a polar orientation -- a "north" end and a "south" end -- and it is attracted to iron objects. But unlike a permanent magnet, in an electromagnet you can alter the orientation of the poles. If you reverse the flow of the current, the north and south ends of the electromagnet switch.

This is exactly what a stereo signal does -- it constantly reverses the flow of electricity. If you've ever hooked up a stereo system, then you know that there are two output wires for each speaker -- typically a black one and a red one.

Essentially, the amplifier in a stereo constantly switches the electrical signal, fluctuating between a positive charge and a negative charge on the red wire. Since electrons always flow in the same direction between positively charged particles and negatively charged particles, the current going through the speaker moves one way and then reverses and flows the other way. This alternating current causes the polar orientation of the electromagnet to reverse itself many times a second.

So how does this fluctuation make the voice coil move back and forth? The electromagnet is positioned in a constant magnetic field created by a permanent magnet. These two magnets -- the electromagnet and the permanent magnet -- interact with each other as any two magnets do. The positive end of the electromagnet is attracted to the negative pole of the permanent magnetic field, and the negative pole of the electromagnet is repelled by the permanent magnet's negative pole. When the electromagnet's polar orientation switches, so does the direction of repulsion and attraction. In this way, the alternating current constantly reverses the magnetic forces between the voice coil and the permanent magnet. This pushes the coil back and forth rapidly, like a piston.

In summary, the stereo amplifier sends alternating (positive and negative) current through an electromagnet called the voice coil. This voice coil is near a permanent magnet, causing the voice coil to vibrate. The vibrating voice coil causes the cone to vibrate, which in turn causes the air to vibrate.

In this experiment you will be given some simple directions for making a speaker. After making this speaker, you will be encouraged to try and design your own speaker, and perhaps make a speaker of your own design for extra credit.

MATERIALS:

§         Drill with a ¼” drill bit

§         Craft Stick

§         Paper for Cone (various types)

§         Paper Template for Cone

§         Glue

§         2” common nail

§         Insulated Copper Wire

§         Connector Leads

PROCEDURE:

1.      Drill a ¼” inch hole in the center of a craft stick.

2.      Cut the cone out of paper using the template given to you by your teacher. Remove the area formed by the triangle of 1-2-3.

3.      Reinforce the area behind line 6 and 7 by placing a piece of tape behind them.

4.      Using a sharp knife, slit along line 6 and 7. Roll the circle into a cone by bringing 5 to meet 3 and 4 to meet 2.

5.      Glue the magnet to the stick so that its center is aligned with the center of the hole.

6.      Slide the craft stick through slots 6 and 7 and line up the hole in the craft stick with the center of the cone.

7.      Wrap the nail with the magnet wire. Start 1” from the head of the nail.

8.      Wrap the nail with wire one layer up to the head of the nail.

9.      Wrap a second layer back to the starting point.

10. Secure the wire with tape, making sure that you have approximately 4” of extra wire left on the leads of wire.

11. The tip of the nail should be resting in the center of the cone. Push the nail through the back of the cone so that the head of the nail sits in the middle of the craft stick.

12. Attach a 1/8” earphone jack to the lead wires.

13. Hook this up to your stereo, and listen closely for music. This apparatus would cause it to possibly overheat.

DISCUSSION QUESTIONS

1.      What do you think happens when a speaker “blows”?

2.      Can speakers work under water?  If so, are there any special design features that you might include?

3.      Would painting the cone of a speaker be a good idea?

4.      Use a dictionary to define the word Amplify. How do you think an amplifier works in a stereo system? Why would some people add an external amplifier to a stereo system?

5.      Could adding an amplifier be bad for a stereo system?

6.      What part of the speaker is like the tympanic membrane in humans?

7.      What part of the speaker is most like the anvil-hammer-stirrup system?

8.      How do you think stereo headphones work?